At the moment of their first breath, a baby's lungs are collapsed and filled with fluid. The pressures needed to open such lungs, and keep them open, are several times that of a normal breath until the fluid is displaced and the lungs have filled with air. To generate these large pressures, the baby must have strong respiratory muscles, as well as a chemical called surfactant in their alveoli. Surfactant reduces the surface tension of the fluid within the alveoli, preventing the alveolar walls from sticking to each other, like coasters to coffee cups when there is water between them.
Neonates have difficulty in opening their lungs and keeping them open. Reasons for this include:
a) Weak respiratory muscles and low surfactant levels. This means that they cannot generate enough pressure to open the lungs and, should they be resuscitated, tire quickly with the effort of keeping open alveoli lacking in surfactant.
b) Underdeveloped internal tissue structure to support the alveoli.
c) Slower clearance of lung fluid. In very premature neonates, fluid may continue to be secreted in the alveoli even after birth.
d) A soft chest wall, horizontal ribs, and a flatter diaphragm contribute to reduce the inspiratory capacity.
e) The mixing of oxygenated and deoxygenated blood raises blood pressure in the lungs, increasing fluid movement from the blood vessels into the lung tissue. The reduced blood oxygen level starves tissue of oxygen and further weakens respiratory muscles.
f) Weak neck muscles and a lack of neck fat reduce upper airway stability so that collapse on inspiration may occur.
g) Collapsed, damaged alveoli secrete proteins that reduce surfactant function.
To alleviate this it is known to apply Positive End Expiratory Pressure (PEEP) during respiration, resuscitation or assisted respiration (ventilation). In applying PEEP, the neonate's upper airway and lungs are held open during expiration against a pressure that stops alveolar collapse. Lung fluid is pushed back into the circulating blood, alveolar surfactant is conserved, and a larger area of the lung participates in gas exchange with the blood. As blood oxygenation and carbon dioxide removal improves, more oxygen is delivered to growing tissues, while less oxygen and energy is consumed by respiratory muscles. In the case of resuscitation or ventilation the pressure is varied between a Peak Inspiratory Pressure (PIP) and the PEEP value until the patient/infant is breathing spontaneously.
In order to provide the PEEP across a range of flow rates, some method is required to regulate the pressure. It is known in the art to provide a valve near the infant, which actuates at a level of pressure (ie: the PEEP value) to allow the gases to vent externally.
Such valves may employ a spring-loaded valve, which in turn requires the use of high quality springs, which have been individually tested to give a high tolerance spring constant in order to ensure that it actuates at a value substantially that of the maximum safe pressure. Both the manufacture and testing of such a spring necessitates that its cost will be correspondingly high. Accordingly it would be advantageous to provide a pressure relief valve for a breathing assistance system which did not involve the use of such a high tolerance spring.
Also such valves are known to have substantial variation of the relief pressure with flow rate. For example as seen in FIG. 5 the delivered pressure is shown for a range of valves. Over a given range of flow rates shown in the graph 50 of FIG. 5, a variable orifice valve as shown by line 52 gives a wide range of delivered pressure. An improvement on this is a prior art umbrella valve (for example the “umbrella check valve” manufactured by Vernay Laboratories Inc. shown in FIGS. 4a & 4b) which delivers a lower variation in delivered pressure, as shown by line 54. However in all cases the variation in delivered pressure of prior art valves would desirably be reduced for this application.